A Gameboy Advance SP trades in for $20 base amount at Gamestop. A DS Lite trades in for $60 base amount at Gamestop. An original DS (phat) trades in for only $20 base amount at Gamestop.
Until 8/31, there is a special going on where you get a bonus $20 credit when you trade in eligible systems. All three of these are eligible.
There is also a coupon floating around (check the Video Game Deals forum) for bonus $30 credit when you trade in an eligible system. It doesn't stack with the $20 bonus. The GBA SP is not eligible for the $30 coupon.
There was another special going on where trading in a GBA SP specificly let you get a new DS for $89.99, but since that's the same as the bonus $20 promotion, I think it's gone. With the ones I described above, you get store credit for the trades, which you can keep and spend when you want to. If you need to make up the difference, check the trade-in values listed in this thread:
http://www.cheapassgamer.com/forums/showthread.php?t=191324
Some DS games are getting a surprisingly high amount.
You can trade broken systems and still get the bonus $20, but the refurb fee is half of the base trade value. You can't use the $30 coupon when trading a broken system.
At my local Gamestop, a used DS Lite is $109.99, while new ones are $129.99, same as everywhere. If you like Pokemon, some slower locations still have the new Pokemon DS Lite bundle in stock as well.